1. Field of the Invention
This invention relates to a magnetoinductive flowmeter serving to measure the volume flow of a medium traveling through a measuring tube and incorporating a magnet for generating a magnetic field that permeates the measuring tube with a magnetic-field component perpendicular to the direction of flow, a first electrode and a second electrode for collecting a voltage induced in the medium, and an amplifier to which the voltage collected by the electrodes is fed. The invention also relates to a method for operating a magnetoinductive flowmeter serving to measure the volume flow of a medium traveling through a measuring tube and incorporating a magnet for generating a magnetic field that permeates the measuring tube with a magnetic-field component perpendicular to the direction of flow, a first electrode and a second electrode for collecting a voltage induced in the medium, and an amplifier to which the voltage collected by the electrodes is fed.
2. The Prior Art
Magnetoinductive flowmeters and methods for operating magnetoinductive flowmeters of the type referred to have been well known for some time and have been employed in a variety of different fields of application. The underlying concept of a magnetoinductive flowmeter for measuring the volume flow of a medium traveling through a measuring tube goes all the way back to Faraday who in 1832 proposed applying the principle of electrodynamic induction for measuring flow rates.
According to Faraday's Law of Induction, a medium that contains charge carriers and flows through a magnetic field will produce an electric field intensity perpendicular to the direction of flow and perpendicular to the magnetic field. A magnetoinductive flowmeter employs Faraday's Law of Induction whereby a magnet, generally consisting of two magnetic poles, each with a field coil, generates a magnetic field that contains a magnetic field component perpendicular to the direction of flow in the measuring tube. Within that magnetic field, each volume element of the medium flowing through the magnetic field and containing a certain number of charge carriers, will contribute the field intensity generated in the volume element concerned to the voltage collected by way of the electrodes.
In conventional magnetoinductive flowmeters, the electrodes are designed either for conductive coupling with the flowing medium or for capacitive coupling with the flowing medium. As a salient feature of magnetoinductive flowmeters, the measured voltage is proportional to the flow rate of the medium averaged across the diameter of the measuring tube. In other words, the measured voltage is proportional to the volume flow.
In a practical flow-measuring operation, the magnetic field in a magnetoinductive flowmeter is generally reversed in an alternating time sequence. The prior art shows different approaches to do that. For example, magnetoinductive flow measuring can be performed by means of an alternating field, in which case the field coils of the magnet are typically connected directly to a sinusoidal 60 Hz A.C. line source. However, transformation-related noise or line interference potentials can easily compromise the flow-generated voltage between the measuring electrodes.
In recent times, magnetoinductive flowmeters have generally been operated with a switched constant field. To create a switched constant field, the field coils of the magnet are fed a current having essentially a square-wave pattern with periodic polarity switching. Also possible, however, is the use of a pulsating constant field that is produced by only periodically feeding to the field coils of the magnet a square-wave current having the same polarity. However, a method whereby the field current is periodically polarity-reversed, producing a periodically alternating magnetic field, is preferred because switching the polarity of the magnetic field suppresses interference potentials such as electrochemical noise.
The voltage between the measuring electrodes is proportional to the flow rate and is usually quite small, typically in the microvolt range. That voltage is measured at the highest possible resolution (approx. 100 mV), with the measuring frequency of the more common magnetoinductive flowmeters employing the switched constant-field principle being in the 1 to 100 Hz range.
In conventional magnetoinductive flowmeters, the voltage collected at the electrodes is generally fed to an amplifier before the amplified, flow rate-dependent voltage signal undergoes further processing. Frequently used amplifiers are of the differential amplifier type. The amplified voltage signal is typically evaluated by means of a microprocessor, meaning that, before the voltage signal is sent to the microprocessor, it requires analog-to-digital conversion by means of an analog-to-digital converter.
To permit and perform the calibration of a magnetoinductive flowmeter of the type described above, it is important for the amplification path, i.e. the path traveled by the voltage signal via the amplifier to the evaluation unit, to work reliably. At the very least, the system must alert the operator of the magnetoinductive flowmeter to any deviation from calibrated operation that necessitates recalibration.